Background: The human tongue muscle genioglossus (GG) is active in speech, swallowing, respiration, and oral transport, behaviors encompassing a wide range of tongue shapes and movement speeds. Studies demonstrate substantial diversity in patterns of human GG motor unit activation, but whether this is accompanied by complex expression of muscle contractile proteins is not known. Purpose: The authors tested for conventional myosin heavy chain (MHC) MHCI, MHCIIA, MHCIIX, developmental MHCembryonic and MHCneonatal and unconventional MHC[alpha]cardiac, MHCextraocular, and MHCslow tonic in antero-superior (GG-A) and posterior (GG-P) adult human GG. Method: SDS-PAGE, Western blot, and immunohistochemistry were used to describe MHC composition of GG-A and GG-P and the prevalence of muscle fiber MHC phenotypes in GG-A. Results: By SDS-PAGE, only conventional MHC are present with ranking from most to least prevalent MHCIIA greater than MHCI greater than MHCIIX in GG-A and MHCI greater than MHCIIA greater than MHCIIX in GG-P. By immunohistochemistry, many muscle fibers contain MHCI, MHCIIA, and MHCIIX, but few contain developmental or unconventional MHC. GG-A is composed of 5 phenotypes (MHCIIA greater than MHCI-IIX greater than MHCI greater than MHCI-IIA greater than MHCIIX). Phenotypes MHCI, MHCIIA, and MHCI-IIX account for 96% of muscle fibers. Conclusions: Despite activation of GG during kinematically diverse behaviors and complex patterns of GG motor unit activity, the human GG is composed of conventional MHC isoforms and 3 primary MHC phenotypes.